The expression of the immediate early genes of herpes simplex virus is regulated by the assembly of a multiprotein complex which contains both cellular and viral factors. Of these, the mammalian C1 transcription factor plays a critical role in the formation of the stable complex. To understand the nature of the interactions which are important for the regulation of HSV gene expression, the domains of the C1 factor which interact with the components of the complex have been mapped. In addition, genetic screens were designed to isolate additional cellular proteins which may play a role in this regulation. These screens have resulted in the isolation of several polypeptides including subunits of the mammalian RNAPII, suggesting that the C1 factor may activate HSV IE genes via direct interaction with the polymerase. These studies are continuing to characterize the specific interactions of these factors and the biochemical mechanism of HSV IE gene regulation. The C1 factor consists of a novel family of polypeptides which are derived from a common precursor by site-specific proteolysis. Studies designed to identify the C1-specific protease have shown that the C1 factor contains an autocatalytic processing domain. Continuing studies focus upon the characterization of this enzymatic activity and the role of the processing in the regulation of the functions and specific activity of the C1 factor. As site-specific proteolysis plays a significant role in many basic cellular and viral processes, a genetic screen was developed for the isolation and characterization of novel site-specific proteases. Using the model proteases encoded by the human immunodeficiency virus (HIV) and herpes simplex virus (HSV), a system was developed which enables the isolation of novel site-specific proteases. In addition, this system provides a new method for the isolation and development of novel antiviral therapeutics